In aged, subfertile mares embryonic losses may occur very early prior to detection with ultrasonography.
When monitored day two to day 14, embryonic loss was 62-73 percent in subfertile mares compared to 0-9 percent in normal mares.
In cases where embryos were recovered early (from day seven to day 10) significantly fewer embryos were recovered from subfertile
(commonly barren) mares than from normal mares.
From studies at the Northwest Equine Reproduction Laboratory, it was determined that early embryonic loss in aged mares was
the failure of the embryo.
Woods transferred embryos from older to younger mares. They did not survive. The conclusion was that of potentially 'bad
eggs', not the health of the older uterus, or the endocrine system, but the embryos themselves.
"The age-related component to increased embryonic loss in mares, which is very similar to what is seen in women, seems to
be associated with decline in oocyte quality or defective oocytes," Ball explains.
"There is not a lot of information on exactly what the defects are at the cellular level, and how they occur," Vanderwall
says. "Carnevale did one study using microscopy. She did see some differences morphologically between oocytes from older
mares compared to oocytes from young ones. She was not able to determine where the structural changes were in the oocytes
from the older mares, or whether the structural changes she saw were directly related to the lower quality of those eggs from
those older mares. She wasn't able to draw a line between those two. That remains to be determined. She saw differences
between the eggs of young and old mares, but whether or not those differences are really the cause for lower quality of the
eggs from old mares has not been determined.
"Another theory of why aged mare eggs are less viable is at the chromosomal level. Factors considered are either gross chromosomal
abnormalities, missing chromosomes, extra chromosomes, or potentially defects at the DNA molecular level. There is no confirming
data in the horse to document that there are chromosomal problems that are the underlying problem in those old mare eggs."
There is increasing data that in these older mares as those embryos are developing, they may have a delay in their embryonic
"That has direct implications for things like embryo transfer," Vanderwall suggests. "For years we would repeatedly flush
the embryos from the mare's uterus on day seven after ovulation. The day seven embryo of the older mare was possibly defective
and delayed in development. It's possible that although according to the calendar she was seven days post ovulation, her embryo
biologically was a day or two delayed in its development, and therefore did not result in a viable pregnancy due to its defective
"From the standpoint of overcoming fertility problems in these older mares, although the oocyte quality is a big issue in
these aged mares, it is also possible that the oviduct plays a role in the embryo's demise. For some mares, and often it's
the older mares, they've got abnormalities of the oviduct (or the uterus), that precludes the mare either herself from getting
pregnant and maintaining the pregnancy, or even precludes the ability to flush an embryo out for transfer."
Dr. Kane is a freelance writer for equine topics and senior nutritionist with Stuart Products, Inc., Bedford, Texas. He holds
a Ph.D in equine physiology and nutrition from the University of Kentucky.